The 'Action of 4 April 1941' was a naval engagement fought during the 'Battle of the Atlantic' between a German commerce raider and a British armed merchant cruiser (4 April 1941).

In this action a German commerce raider, Thor (known to the Germans as Schiff 10 and HSK-4 and to the British as Raider 'E'), encountered the British armed merchant cruiser Voltaire and sank her after a short engagement. The German crew rescued the British survivors, some of whom were repatriated and therefore give an account to the Admiralty, which then accepted that German commerce raiders were too well-armed for converted ocean liners equipped nothing more than obsolete guns, but nothing better was available to the Royal Navy until later in the war.

During the 1930s, the Kriegsmarine had subsidised banana plantations in Central America to order banana boats built by German yards to a standard which allowed their quick conversion for navy use. With a speed of 17 or 18 kt, such ships were faster than average freighters and sat lower in the water, which made them smaller targets. The Oldenburg-Portugiesische Dampfschiffahrtsgesellschaft’s 3,862-ton ship Santa Cruz was requisitioned by the Kriegsmarine and converted into a merchant raider by her builder, Deutsche Werft A.G. The ship was armed with six 150-mm (5.91-in) guns, one 60-mm (2.36-in) gun, two twin 37-mm Flak guns, four twin 20-mm Flak cannon and four 533-mm (21-in) torpedo tubes. The raider also carried one Arado Ar 196A-1 floatplane for reconnaissance, and had a complement of 349 persons.

Voltaire, which had been constructed by Workman & Clark in 1923 as a passenger liner of the Lamport and Holt line, carrying passengers and freight between New York and Buenos Aires, displaced 13,245 tons, had a speed of 14.5 kt and was operated by a crew of 269 persons. Employed initially after impressment as a troopship and then as an accommodation ship at Scapa Flow in the Orkney islands group, she was later sent to Wallsend for adaptation as an armed merchant cruiser. In this role, Voltaire carried eight 6-in (152.4-in) and two 3-in (76.2-mm) naval guns, including at least one anti-aircraft mount. Voltaire patrolled the Mediterranean Sea enforcing contraband regulations, then steamed to Halifax, Nova Scotia, to escort convoys across the Atlantic. Having refitted at St John, New Brunswick, Voltaire later sailed for Trinidad and was then sent to Freetown in Sierra Leone to look for commerce raiders.

On 6 June 1940, under the command of Kapitän Otto Kähler, Thor sailed on a raiding voyage to the central and south Atlantic, disguised as the Soviet merchant ship Orsk: the raider departed Norway and passed through the Denmark Strait on 16 June. On 1 July, masquerading as a Yugoslav freighter, she intercepted and seized the Dutch Kertosono, which was sent to a German base in occupied France as a prize. On 7 June the British Delambre was overhauled and sunk, and just two days later the Belgian Bruges was sunk. On 14 July, Gracefield was forced to heave to and was then sunk. None of these ships got off a distress report. On 16 July, the British ship Wendover fought back and managed to transmit a report before being sunk by gunfire and torpedoes after her crew had been taken prisoner, bringing Thor's total of prisoners to more than 200 men. The Dutch Tela was captured on the next day, and the seizure of her cargo of foodstuffs alleviated the difficulty of feeding the prisoners.

On 28 July, Thor encountered the 22,180-ton British armed merchant cruiser Alcantara not far from the Trinidade islands group, some 690 miles (1110 km) off the coast of Brazil, to which the islands belong. Radio reports and overdue ships had by this time indicated to the British that a raider was operating in the Atlantic Ocean, and it was for this reason that Alcantara had been despatched to investigate. Thor was armed with four main guns which considerably outranged the armament of eight older 6-in (152.4-mm) guns carried by Alcantara but Thor was slower and therefore could not escape the armed merchant cruiser. In the resulting engagement, which lasted for about four hours, Thor inflicted serious damage and received only two hits in return, and Alcantara broke off and headed slowly toward Brazil. Thor moved away to the south under cover of a smokescreen, to complete repairs and rendezvous with a supply ship. On 26 September, in the south Atlantic, the Norwegian Kosmos III was sunk, followed on 8 October by the British Natia. Now carrying more than 360 prisoners, Thor on 9 November made rendezvous with the blockade-runner Rio Grande, which transferred supplies and took off most of the prisoners.

On 5 December Thor met the 20,122-ton armed merchant cruiser Carnarvon Castle to the east of South America. The German raider again outgunned and outranged the British ship, which was armed with eight obsolete 6-in (152-mm) guns, but was too slow to outrun Carnarvon Castle, which was capable of 188 kt. Thor inflicted so much damage with its stern armament that Carnatrvon Castle had to make for port for repairs. Thor made rendezvous with the 'pocket battleship' Admiral Scheer and a captured freighter on 25 December, from which a large quantity of supplies was garnered. Several British cruisers were concentrated off the Plate river and Rio de Janeiro by this time, but Thor encountered few ships apart from German supply vessels for three months. Thor communicated to Germany important information of her superiority over British armed merchant cruisers, and this gave the crews of other raiders greater confidence against the British ships. On 25 March 1941, the passenger liner Britannia was sunk and the occupants left in lifeboats because Thor had intercepted an RRR (raider report) transmission from Britannia and a reply from what the captain took to be a nearby British warship. Later in that same day Thor sank the Swedish ship Trolleholm.

Early in April 1941, as noted above, the armed merchant cruiser Voltaire sailed from Trinidad in the Caribbean for Freetown, Sierra Leone, with instructions that during her passage she was to search the area to the west of the Cape Verde islands group for commerce raiders.

On 4 April Thor was back in the area to the north-east of Brazil, disguised as a Greek freighter, and at 06.15 her look-outs reported smoke on the horizon to port. The ship Voltaire, under the command of Captain James Blackburn. Kähler turned toward the vessel, not realising that it was an armed merchant cruiser, and the British assumed that they had met a small freighter and closed to check. Voltaire sailed nearly straight at Thor from a distance of about 15,000 yards (13715 m), and at 06:45 and a range of about 9,000 yards (8230 m), Kähler had the forward 150-mm (5.91-in) gun fire a warning shot across the ship’s bow. Kähler was surprised when the ship returned fire and replied with two guns. On this occasion, the armed merchant cruiser was well within range and on a converging course when the engagement began. The German gun crews hit Voltaire with their opening salvo of four shells, destroying the radio room before an RRR report could be sent, and wrecking Voltaire's fire-control system. Voltaire was set on fire within four minutes.

With her fire-control system out of action, Voltaire could not co-ordinate the fire of her 6-in (152.4-mm) guns and her fire became slow and inaccurate. At 07.15, Voltaire suffered a hit on her steering gear and began to turn in circles at a speed of 13 kt. Blackburn abandoned the bridge when it was ablaze and went to the stern to command one of the two remaining 6-in (152.4-mm) guns, but managed to hit only the top of Thor's mast. Kähler fired two torpedoes from 7,000 yards (6400 m), but neither hit Voltaire. At about 08.00 Blackburn gave the order to abandon ship, and at 08.35 Voltaire rolled over and sank. Thor rescued 189 survivors; 76 of the British ship’s crew had been killed or died of wounds. In 55 minutes Thor had fired 724 rounds, inflicted many hits, and in return had suffered only minor damage.

Rather than leave the crew to be rescued, Kähler searched the area for five hours despite the fact that other British ships might have appeared. Thor then headed to the north, changed disguise, rendezvoused with the tanker Ill,and then set course for Europe. German wireless broadcasts made much of the sinking of Voltaire but did not disclose the identity of the raider. After the Germans had announced the sinking of Voltaire, the Admiralty sent the Canadian armed merchant cruiser Prince David to search, and she found wreckage half-way between Trinidad and the Cape Verde islands group. The British remained ignorant of the details of the engagement until survivors had been repatriated from Germany.

On 16 April Thor sank a Swedish freighter, her eleventh success in a 'bag' of 83,301 tons of shipping and one armed merchant cruiser. Thor entered the Bay of Biscay on 23 April and slipped up the English Channel to reach Hamburg on 30 April.

The Admiralty understood the pointless nature of using obsolete guns to arm slow and old liners as trade protection vessels, but lacked the modern guns and ships to take on German commerce raiders until later in the war. Thor made another cruise in 1942 but was burned out in Yokohama when the supply ship Uckermarck lying alongside, suffered an explosion and fire while her fuel tanks were being cleaned.